Circuits with feedback loops are widely used in various applications, such as radiofrequency (RF) and microwave transmitters. Various digital modulation schemes are employed.
Digital linear modulation increases spectral efficiency when compared to non-linear modulation schemes. Transmitter non-linearity may cause transmitted output power to enter adjacent channels, and the amount of power in an adjacent channel is function of the linearity of the transmit path. Linearization of a power amplifier can be done using Cartesian feedback loops, which are capable of providing significant linearity in narrow bandwidth systems.
Cartesian feedback loops tend to be implemented using integrated circuits. Power control may be provided to Cartesian feedback loops for various reasons. For example, a transmitter may be operated at different power levels depending on various constraints.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.